1. Field of the Invention
The present invention relates to a polyimide seamless belt used for an image forming apparatus using electrophotography, such as a photocopier, and a printer, and to a production method thereof.
2. Description of the Related Art
Conventionally, a polyimide seamless belt has been used for various use, as a member of an image forming apparatus utilizing electrophotography. Examples of use thereof include a fixing belt, a transfer belt, and a paper conveyance belt. Particularly, a polyimide seamless belt is used as an intermediate transfer belt or a base of a fixing belt (see, for example, Japanese Patent (JP-B) No. 3708867).
Various methods have been known as a production method of the aforementioned polyimide seamless belt used in an electrophotographic image forming apparatus. The methods that have been already known include centrifugal forming where an internal surface of a mold is coated with a coating material and the coating material is flow casted to yield a shaped product, and a method where a coating material (referred to as a polyamic acid composition liquid hereinafter) is prepared by synthesizing polyamic acid, which is a precursor of polyimide, in an organic polar solvent, and adding conductive carbon black to the resultant, and an internal or external surface of a mold is coated with the coating material, and the coating material is flow-casted, and the resultant is dried and imidized using a hot air furnace (see, for example, JP-B Nos. 5079354, and 4406782).
Such a belt is required to have various properties, such as heat resistance, and surface qualities, depending on use thereof, as well as mechanical properties, and electric properties. It has been known that the properties are influenced by firing conditions at the time of drying and imidization. A polyimide seamless belt having high strength and high resistance to heat can be attained by sufficiently removing the residual solvent through heating.
If a thick polyimide seamless belt is attempted to be produced to enhance the strength, a solvent permeability becomes poor depending on a type of polyamic acid for use due to a molecular structure thereof, and swelling or a defect in an appearance, such as rough surface (texture like zest of a citrus fruit) may occur.
In this case, a thick seamless belt is formed by typically applying a coating material to a metal mold, and taking time to perform an imidization process so as not to impair an appearance of a resulting seamless belt. Alternatively, as disclosed in JP-B No. 3708867, internal surfaces of a metal mold is coated with a polyamic acid composition liquid, the polyamic acid composition liquid is casted, the composition liquid is dried at 130° C. or lower, and then a resultant is taken out from the mold. The dried product is then placed in another mold for imidization firing in the manner that a surface thereof to be in contact with the air is reversed during drying and during imidization, to thereby produce a thick seamless belt. When a seamless belt is produced in the aforementioned manner, however, it takes a lot of trouble, as the mold for use needs to be changed when dried and when imidized, and a cost for equipment is increased, as a number of steps and processes to be performed is increased.
Moreover, Japanese Patent Application Laid-Open (JP-A) No. 2011-002713 discloses that electric resistivity is secured with a minimum amount of acidic carbon black, by using quaternary ammonium salt as a cationic surfactant, to thereby stably homogeneously disperse the acidic carbon black in a polyimide precursor. Moreover, JP-A No. 2002-148957 discloses an intermediate transfer member containing 20% by weight to 30% by weight of oxidized carbon black, and lists, as examples of the oxidized carbon black, PRINTEX150T (pH: 4.5, volatile matter: 10.0%) manufactured by Evonik Industries AG, Special Black 350 (pH: 3.5, volatile matter: 2.2%) manufactured by Evonik Industries AG, Special Black 100 (pH: 3.3, volatile matter: 2.2%) manufactured by Evonik Industries AG, Special Black 250 (pH: 3.1, volatile matter: 2.0%) manufactured by Evonik Industries AG, Special Black 5 (pH: 3.0, volatile matter: 15.0%) manufactured by Evonik Industries AG, Special Black 4 (pH: 3.0, volatile matter: 14.0%) manufactured by Evonik Industries AG, Special Black 4A (pH: 3.0, volatile matter: 14.0%) manufactured by Evonik Industries AG, Special Black 550 (pH: 2.8, volatile matter: 2.5%) manufactured by Evonik Industries AG, Special Black 6 (pH: 2.5, volatile matter: 18.0%) manufactured by Evonik Industries AG, Color Black FW200 (pH: 2.5, volatile matter: 20.0%) manufactured by Evonik Industries AG, Color Black FW2 (pH: 2.5, volatile matter: 16.5%) manufactured by Evonik Industries AG, Color Black FW2V (pH: 2.5, volatile matter: 16.5%) manufactured by Evonik Industries AG, MONARCH 1000 (pH: 2.5, volatile matter: 9.5%) manufactured by Cabot Corporation, MONARCH 1300 (pH: 2.5, volatile matter: 9.5%) manufactured by Cabot Corporation, MONARCH 1400 (pH: 2.5, volatile matter: 9.0%) manufactured by Cabot Corporation, MOGUL-L (pH: 2.5, volatile matter: 5.0%) manufactured by Cabot Corporation, and REGAL 400R (pH: 4.0, volatile matter: 3.5%) manufactured by Cabot Corporation. Two of these oxidized carbon black may be used in combination.
However, a surfactant is typically decomposed, as the surfactant is exposed to temperature exceeding 300° C. Polyimide is attained by heating polyamic acid composed of tetracarboxylic dianhydride and a diamine component to perform a dehydration condensation reaction. The temperature for the heating is 300° C. to 450° C., and thus it is hardly believed that the surfactant inside the formed belt has a function to directly reduce electric resistance. When it is attempt to form a belt having a surface resistivity Log ρs of 5.0 or less using the surfactant disclosed in JP-A No. 2011-002713, moreover, formation of a conduction path is inhibited by a functional group (e.g., a carboxyl group, a ketone group, a lactone group, and a hydroxyl group) containing oxygen present on a surface of acidic carbon black. Therefore, it is necessary to significantly increase the amount of the acidic carbon black added, which impairs mechanical strength of the belt.
Moreover, there is no assumption or intention for the technology disclosed in JP-A No. 2002-148957 to use high-structure carbon black. Accordingly, the technology naturally does not aim to improve dispersibility of carbon black when a coating liquid containing the high-structure carbon black which can exhibit conductivity with a small amount, and polyamic acid.
When a high-structure carbon black dispersion liquid, which an exhibit conductivity with a small amount of the carbon black, is produced in order to reduce an amount of the carbon black in a coating liquid, most types of the carbon black are neutral or alkaline, and have poor solubility to a polar solvent that is a solvent for polyamic acid. Therefore, there are problems that the amount of the carbon black is reduced to reduce viscosity and solid content, when a dispersion liquid or a coating material is produced.